A process of this type is called a "sealing" process because, in some instances, the effect of the process has been theoretically ascribed to sealing pores that exist in the primary coating. The evidence for such pores is generally regarded as very strong for coatings formed by anodizing aluminum, for example, but the term "sealing" is now applied to any liquid composition that can be contact with a pre-existing coating, particularly a conversion coating, over a metal surface in order to improve the corrosion resistance of the object including the metal surface and all of its protective coating(s) and to any process that thus improves the corrosion resistance, irrespective of whether any pores in the primary coating are actually sealed or even existed at the time of treatment. Alternative names for what is herein called "sealing" in this sense include "passivating", "final rinsing", "post-rinsing", and the like. A sealing treatment according to this invention is especially advantageous for surfaces that are not intended to receive any further protective organic based coating such as paint or the like, but is also useful for substrates that are to be further protected in this way.
Numerous sealing compositions for a variety of primary coatings are known in the prior art, but, particularly for aluminum substrates that are not to receive any substantial protective coating of paint or a similar material containing an organic binder, still further improvements in corrosion resistance are desirable. Accordingly, a major object of this invention is to provide sealing compositions and processes that, in combination with known primary coatings, produce improved corrosion resistance, especially in the absence of organic protective coatings. Other alternative or concurrent objects are to minimize adverse environmental impact compared with related previously used sealers, which often contained hexavalent chromium or other materials capable of readily damaging the environment, and to provide more economical treatments without diminishing the corrosion protection achieved. Other objects will be apparent from the description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, "parts of", and ratio values are by weight or mass; the term "polymer" includes "oligomer", "copolymer", "terpolymer" and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not necessarily preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word "mole" means "gram mole", and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; and the terms "solution", "soluble", "homogeneous", and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18-25.degree. C.